The influence of dietary fat types, individual dietary fatty acids and possible dietary fat-selenium interactions on development of spontaneous mammary tumors in virgin female C3H mice is being studied. This is being done by including, as 10 percent of the total diet, fats from various sources, singularly and in computerized combinations, to yield the lowest possible correlation ratios between fatty acids. Additional groups of mice, consuming diets representing plant or animal fats or various degrees of fatty acid saturation, are being supplemented with 2 ppm sodium selenite in their drinking water. Feed consumption is being measured and the mice are weighed and palpated for mammary tumor development weekly. Rate of tumor growth is recorded. Tissues from mice in each dietary group are analyzed for lipid and selenium content and glutathione peroxidase activity.